In recent years, due to the spread of portable devices, such as notebook computers and mobile phones, there is an increasing demand for batteries that are power supplies of the portable devices. Especially, there is an increasing demand for secondary batteries which are small in size, light in weight, high in energy density, and repeatedly rechargeable. As such battery, a lithium ion secondary battery using a nonaqueous solvent as an electrolytic solution has been intensely researched and developed.
In accordance with the increase in function of the portable devices, the energy of the lithium ion secondary battery is increasing. In proportion to this, the amount of heat which may be generated at the time of a potential malfunction is also increasing.
For ease of handleability, such secondary battery is stored in a resin container and sold as a battery pack. Used as an exterior container constituting the battery pack is a container (see PTL 1, for example) formed by polycarbonate resin mixed with a halogen-based flame retardant or a container (see PTL 2, for example) formed by a resin composition containing polyphenylene ether, styrene-based resin, and a phosphoester-based flame retardant.
In order to further increase the flame retardancy of the above resin composition, proposed is to mix the resin composition with inorganic hydroxide, such as magnesium hydroxide (Mg(OH)2), aluminum hydroxide (Al(OH)3), or dawsonite (NaAl(OH)2CO3) (see paragraph 0023 in PTL 2, for example). When each of the magnesium hydroxide and the aluminum hydroxide is heated, it absorbs ambient heat and discharges water (H2O). Therefore, combustion heat is reduced by this heat absorbing action. Thus, a flame-retardant effect can be exerted.
As another method for suppressing the temperature increase of the surface of the battery pack by utilizing the heat absorbing action when the malfunction has occurred, proposed is a method for introducing a polymeric material in the battery pack to utilize melting latent heat generated when the polymeric material melts (see PTL 3, for example).
Moreover, to prevent moisture from intruding from the outside, proposed as an exterior material of the battery pack is a stack body formed by stacking an exterior resin layer, a metal layer, and an inner resin layer in this order (see PTL 4, for example).
Proposed is that in a vacuum heat-insulating container configured to store a plurality of sodium-sulfur unit cells, to prevent a second accident caused by reaction heat generated at the time of break of the unit cell, granular fireproofing agents, such as fire extinguishing sands or lightweight aggregates, are filled in the container or a heat insulating board made of glass wool or ceramic wool is used in the container (see PTL 5, for example).